Re: [PATCH 6.10 000/809] 6.10.3-rc3 review

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Wed, 2024-08-07 at 01:24 +0200, Thomas Gleixner wrote:
> Cc+: Helge, parisc ML
> 
> We're chasing a weird failure which has been tracked down to the
> placement of the division library functions (I assume they are
> imported
> from libgcc).
> 
> See the thread starting at:
> 
>  
> https://lore.kernel.org/all/718b8afe-222f-4b3a-96d3-93af0e4ceff1@roeck-us.net
> 
> On Tue, Aug 06 2024 at 21:25, Vlastimil Babka wrote:
> > On 8/6/24 19:33, Thomas Gleixner wrote:
> > > 
> > > So this change adds 16 bytes to __softirq() which moves the
> > > division
> > > functions up by 16 bytes. That's all it takes to make the stupid
> > > go
> > > away....
> > 
> > Heh I was actually wondering if the division is somhow messed up
> > because
> > maxobj = order_objects() and order_objects() does a division. Now I
> > suspect
> > it even more.
> 
> check_slab() calls into that muck, but I checked the disassembly of a
> working and a broken kernel and the only difference there is the
> displacement offset when the code calculates the call address, but
> that's as expected a difference of 16 bytes.
> 
> Now it becomes interesting.
> 
> I added a unused function after __do_softirq() into the softirq text
> section and filled it with ASM nonsense so that it occupies exactly
> one
> page. That moves $$divoI, which is what check_slab() calls, exactly
> one
> page forward:
> 
>     -0000000041218c70 T $$divoI
>     +0000000041219c70 T $$divoI
> 
> Guess what happens? If falls on it's nose again.
> 
> Now with that ASM gunk I can steer the size conveniently. It works up
> to:
> 
>     0000000041219c50 T $$divoI
> 
> and fails for
> 
>     0000000041219c60 T $$divoI
>     0000000041219c70 T $$divoI
> 
> and works again at
> 
>     0000000041219c80 T $$divoI

So just on this, you seem to have proved that only exact multiples of
48 work.  In terms of how PA-RISC caching works that's completely nuts
... however, there may be something else at work, like stack frame
alignment.

> 
> So I added the following:
> 
> +extern void testme(void);
> +extern unsigned int testsize;
> +
> +unsigned int testsize = 192;
> +
> +void __init testme(void)
> +{
> +       pr_info("TESTME: %lu\n", PAGE_SIZE / testsize);
> +}
> 
> called that _before_ mm_core_init() from init/main.c and adjusted my
> ASM hack to make $$divoI be at:
> 
>     0000000041219c70 T $$divoI
> 
> again and surprisingly the output is:
> 
>     [    0.000000] softirq: TESTME: 21

OK, why is that surprising?  4096/192 is 21 due to integer rounding.

> Now I went back to the hppa64 gcc version 12.2.0 again and did the
> same ASM gunk adjustment so that $$divoI ends up at the offset 0xc70
> in the page and the same happens.
> 
> So it's not a compiler dependent problem.
> 
> But then I added a testme() call to the error path and get:
> 
> [    0.000000] softirq: TESTME: 21
> [    0.000000]
> =====================================================================
> ========
> [    0.000000] BUG kmem_cache_node (Not tainted): objects 21 > max 16
> size 192 sorder 0
> 
> Now what's wrong?
> 
> Adding more debug:
> 
> [    0.000000] BUG kmem_cache_node (Not tainted): objects 21 > max 16
> size 192 sorder 0 21
> 
> where the last '21' is the output of the same call which made maxobj
> go
> south:
> 
>  static int check_slab(struct kmem_cache *s, struct slab *slab)
>  {
>         int maxobj;
> @@ -1386,8 +1388,10 @@ static int check_slab(struct kmem_cache
>  
>         maxobj = order_objects(slab_order(slab), s->size);
>         if (slab->objects > maxobj) {
> -               slab_err(s, slab, "objects %u > max %u",
> -                       slab->objects, maxobj);
> +               testme();
> +               slab_err(s, slab, "objects %u > max %u size %u sorder
> %u %u",
> +                        slab->objects, maxobj, s->size,
> slab_order(slab),
> +                        order_objects(slab_order(slab), s->size));
>                 return 0;
>         }
>         if (slab->inuse > slab->objects) {
> 
> I don't know and I don't want to know TBH...

OK, so you're telling us we have a problem with slab_order on parisc
... that's folio_order, so it smells like a parisc bug with
folio_test_large?  Unfortuntely I'm a bit pissed in an airport lounge
on my way to the UK, so I've lost access to my pa test rig and can't
test further for a while.

Regards,

James